The solution currently most frequently adopted (which is the solution to which reference will be made by way of example in the remainder of the present description) envisions that the insertion element is represented by a catheter provided with a balloon part. Once the stent has been brought to the site of implantation by a catheter, it is possible to deploy the stent by distending the balloon. That is, the stent is brought from its radially contracted condition to its radially expanded or extended condition, in which the stent performs the desired action of stenting on the portion of the vessel being treated.
Formerly, it was common to crimp the stent onto the insertion element (typically a catheter) just prior to the implantation intervention. This was carried out by means of an operation which was usually entrusted to a doctor and/or to the staff that performed the implantation operation. Subsequently, the solution that has increasingly gained ground is one in which the stent is mounted on the insertion element as final step, carried out at an industrial level, of the process of preparation of an implantation kit consisting of the stent already mounted directly on the insertion element.
The problems linked to coupling the stents to the corresponding insertion elements are numerous but essentially amount to a number of basic requirements, namely:                the coupling operation must be sufficiently quick and convenient;        the operation must not have adverse effects on the operating characteristics either of the stent or of the insertion element; and        the coupling must be absolutely firm and secure, in such a way as to rule out completely any risk of the stent possibly being undesirably separated from the insertion element, it being necessary for separation to occur (and moreover in a precise and reliable way) only after the stent has been properly positioned and deployed in the site being treated.        
The prior art regarding coupling of stents to corresponding insertion elements is highly extensive. For example, from U.S. Pat. No. 5,725,519 a solution is known in which a tube, pre-installed together with a stent, is fixed to a component having a tension formation. The tube with the stent extends into the cavity of a second component, which is also provided with a tension formation. The hole of the second component is conical and has a tapered cross section in the direction of the first component. By acting on the two components in the direction that causes them to be moved away from one another, it is possible to pass the tube through the conical hole, so reducing the diameter of the stent, and hence bringing about its radial contraction and coupling onto the insertion element consisting of a balloon catheter.
U.S. Pat. No. 5,911,452 (see also EP-A-0 867 156) describes the use of a casing with an inner chamber traversed by a flexible tube. The stent is positioned inside a median portion of the flexible tube and the balloon part of the catheter is inserted into the flexible tube, located inside the stent. A fluid under pressure is injected into the aforesaid chamber which compresses the flexible tube in a circumferential direction, simultaneously compressing the stent so as to grip it (i.e., crimp it) onto the catheter balloon.
From EP-A-0 903 122, a gripping or crimping tool is known formed by a cylindrical element provided with an external thread and by a rotating collar with an internal thread which engages the aforesaid external thread. The neck part of the cylindrical element is subdivided into a series of jaws subjected to a bias in the direction of expansion. The stent, which is set on the balloon catheter and placed inside the aforesaid jaws in an open condition, undergoes a crimping operation when the aforesaid collar is made to rotate and advance towards the jaws.
EP-A-0 873 731 describes a device provided with a set of oscillating-arm parts, as well as an annular or tubular element associated to the aforesaid parts. The element in question has an opening, which is on the whole cylindrical and is uniformly compressible in a radial direction inwards when the arm parts oscillate downwards starting from the intermediate portion. In this way it is possible to crimp the stent onto the insertion element.
EP-A-0 916 318 describes another crimping tool comprising a tapered tube mounted coaxially on the catheter in a position adjacent to the distal end of the stent. The aforesaid tube, which can be peeled off, has a first diameter greater than the diameter of the stent, as well as a second diameter smaller than the diameter of the stent prior to crimping. By sliding the tube onto the stent, a radial force is applied, directed inwards, which is distributed evenly on the circumference.
From EP-A-0 916 319 yet another crimping tool is known comprising supporting elements set at a distance apart, as well as a helical spring applied at one end to a stem and at the opposite end to one of the supports. The stent/balloon catheter ensemble is inserted into the axial cavity of the spring, with the result that the spring carries out crimping of the stent.
U.S. Pat. No. 5,893,852 describes another apparatus comprising a cylindrical casing made up of two co-operating parts. The stent, which is mounted on the balloon, is inserted in a cylindrical cavity the distal part of which has a conical end. The stent is compressed and crimped on the balloon by means of screwing of the two aforesaid parts.
U.S. Pat. No. 5,860,966 describes yet another apparatus comprising a casing with a cylindrical membrane that is closed at both ends so as to form a fluid-tight chamber between the membrane and the casing. A fluid is introduced under pressure into the chamber which acts on the membrane until it brings the membrane into a condition of pressure contact with the stent so as to force the stent into a condition of engagement on the balloon.
From U.S. Pat. No. 5,810,838 a solution is known that is based upon the use of a hollow chamber defining a space designed to be filled with fluid and of a compliant tubular sleeve set inside the chamber with one open end communicating with the outside of the casing. The stent is positioned on the corresponding catheter and inserted into the sleeve. The fluid is pressurized so as to compress the sleeve and the stent radially.
U.S. Pat. No. 6,009,614 describes a device is known which comprises a cylindrical rigid frame inside which an elastic tube is set. The stent, already mounted on the balloon, is inserted in an opening of the frame. The elastic tube is subjected to axial pressure so as to reduce its length and increase its thickness in the radial direction to effect crimping of the stent.
U.S. Pat. No. 6,018,857 describes a tool comprising a grip from which there extends a tube. The stent is set around the tube and is mounted on the balloon by gripping the tool grip and acting in a direction where the grip is pushed away from the catheter. WO-A-00/06052 discloses a crimping tool comprising a stationary plate and a sliding-platform element. A closing plate is hinged to the sliding platform so as to partially overlap the stationary plate in a lowered position. The stent, which is already slightly crimped by hand, is set on the stationary plate starting from a lateral position. The closing plate is displaced into the lowered position so as to withhold the stent in such a way that an external force applied on the closing plate, in combination with the movement of translation of the closing plate itself, produces crimping of the stent.
U.S. Pat. No. 6,024,737 describes a device that has a compressible bend part that can be compressed in a radial direction inwards so as to obtain crimping of the stent and U.S. Pat. No. 6,051,002 describes a device is known which comprises a pair of grips that form a bend part for withholding the ensemble made up of the stent and the catheter. The ends of the bend are displaced in opposite directions, so reducing the radial dimensions of the bend to crimp the stent on the balloon.
Yet other documents envision the possibility of using methods of thermal treatment (so-called “annealing”): in this connection see, for instance, WO-A-99/15106 and U.S. Pat. No. 6,063,092.
Yet other documents specifically tackle the problem of preventing relative sliding between the stent and the catheter. In this connection see, for example, U.S. Pat. No. 5,893,852 (previously cited), U.S. Pat. No. 5,913,871, EP-A-0,855,171, EP-A-0,897,730 and EP-A-0,901,776.